Electrical devices having electrical energy stores (for example batteries or rechargeable batteries) and means for indicating the "low energy store" state (for example a display, bell etc.) are, for example, mobile telecommunications devices (radios) which are based on the frequency and/or time-division multiplex method or the code-division multiplex method and are supplied with electrical power via rechargeable batteries or batteries.
For example, there are thus cordless mobile parts according to the DECT/GAP Standard (Digital European Cordless Telecommunication; see (1): Nachrichtentechnik Elektronik 42 [Telecommunications electronics 42] (1992, January/February/. No. 1, Berlin, DE; U. Pilger: "Struktur des DECT-Standards" [Structure of the DECT Standard], pages 23 to 29; (2): Philips Telecommunication Review, Vol. 49, No. 3, September 1991; R. Mulder: "DECT-Universal Cordless Access System", pages 68 to 73/Generic Access Profile; see ETSI Publication (prETS 300 444, April 1995, Final Draft, ETSI, FR) or mobile radio mobile parts to the GSM Standard (Groupe Speciale Mobile or Global System for Mobile Communication; see Informatik Spektrum 14 [Information spectrum 14] (1991) June, No. 3, Berlin, DE; A. Mann: "Der GSM-Standard--Grundlage fur digitale europaische Mobilfunknetze" [The GSM Standard--The basis of European digital mobile radio networks], pages 137 to 152).
Based on the document Components 31 (1993), Issue 6, pages 215 to 218; S. Althammer, D. Bruchmann: "Hochoptimierte IC's fur DECT-Schnurlostelefone" [Highly optimized ICs for DECT cordless telephones], FIG. 1 shows the basic circuit design of a cordless mobile part MT which is used, for example, the Siemens "Gigaset 951/952" cordless telephone (see telcom report 16 (1993), Issue 1, pages 26 and 27).
The circuit design comprises a radio section FKT, a signal processing device SVE with a signal control section SST designed as a burst mode controller BMC, and a signal conversion section SUT designed as a Codec and AD/DA converter, a clock generator TG, a central controller ZS designed as a microcontroller .mu.c, a BOF interface BSS to an operator interface BOF with a keypad TA, an indicating device AE, an earpiece HK, a microphone MF and a bell TRK as well as a power supply SV with a rechargeable battery (accumulator) AKK, which items are connected to one another in the illustrated manner. The principle of the method of operation of the circuit design is described, for example, in the documented cited above, Components 31 (1993), Issue 6, pages 215 to 218.
Instead of the rechargeable battery AKK, it is also possible to use a battery as an energy store for the cordless mobile part MT. When the stored energy is exhausted, in contrast to the rechargeable battery which is regenerated at the charging station LST, this battery is replaced by a new battery.
In order to charge the rechargeable battery AKK, the mobile part MT forms a functional unit with a charging station LST in such a manner that the rechargeable battery AKK of the mobile part MT is charged, for example, in accordance with the method known from WO94/10782.
If--after such charging--the mobile part MT has been in operation for a relatively long time period and, in consequence, the capacity of the rechargeable battery AKK is virtually exhausted, then--as is generally normal for electrical devices powered by rechargeable batteries--the "low battery" state is indicated. The indication can in this case be produced visually by displaying a rechargeable battery symbol on the indicating device (display) AE of the mobile part MT, and/or audibly by emitting a warning tone via the bell TRK of the mobile part MT. Both the display of the rechargeable battery symbol and the emission of the warning tone should be produced in good time for the user of the mobile part MT in this case so that the mobile part still has a residual operating time until the "rechargeable battery flat" state is reached, which residual operating time can be used by the user of the mobile part MT if required for message transmission (for example continuing a telephone call although the "low rechargeable battery" state is being indicated). On the other hand, neither the display of the rechargeable battery symbol nor the emission of the warning tone should be produced too early, since such premature indication can be misinterpreted by the user of the mobile part MT with regard to the overall operating time (for example overall call time) of the mobile part. It is therefore desirable to comply with an accurately dimensioned residual operating time of the mobile part MT (for example 10 minutes) which is acceptable for the user.
In order to satisfy the requirements quoted above, it is known--for example in the case of the Gigaset 951/952--for the "low rechargeable battery" state to be indicated when the rechargeable battery voltage supplied by the rechargeable battery AKK falls below a value of, for example, 1.15 V for a specified time. Undershooting the voltage value (1.15 V) for the specified time is defined by a voltage range.
Owing to the fact that the rechargeable battery packs have tight tolerances in terms of the rechargeable battery voltage, this procedure is sufficient in the case of mobile parts in which a plurality of rechargeable batteries AKK, so called rechargeable packs, are used.
However, if commercially available Mignon rechargeable batteries are used in the mobile part, it is no longer sufficient for the "low Mignon rechargeable battery" state to be indicated when the rechargeable battery voltage supplied by the Mignon rechargeable battery falls below a specified value U.sub.LB (U.sub.Low Bat =voltage value U at which the "low rechargeable battery" state is indicated) using a U.sub.LB voltage range (undershooting of the voltage value U.sub.LB for a specified time). This situation is explained with reference to FIG. 2.
FIG. 2 shows three discharge voltage profiles 1, 2, 3 of Mignon rechargeable batteries from different manufacturers and three shaded areas 1, 2, 3 which are associated with the discharge voltage profiles , , . The shaded areas , , represent residual operating times (for example residual call times), a first residual operating time RBZ1, a second residual operating time RBZ2 and a third residual operating time RBZ3, of the Mignon rechargeable batteries after the identification (indication) of the "low rechargeable battery" state. The first residual operating time RBZ1 is, for example, set optimally (precise dimensioning and acceptance of the user) for the rechargeable battery type 1 (the U.sub.LB voltage range is passed through optimally with respect to time). The second residual operating time RBZ2 is too long, since, owing to the lower voltage level of the rechargeable battery type 2, the U.sub.LB voltage range is passed through too slowly. The third residual operating type RBZ3 is too short since, owing to the higher voltage level of the rechargeable battery type 3, the U.sub.LB voltage range is passed through too quickly.
The same effect occurs when a pack of rechargeable batteries (rechargeable battery pack) is discharged at different ambient temperatures, since the voltage level is very highly dependent on the temperature.
EP-0 555 012 A2 discloses a method for indicating the discharge level of energy stores (for example batteries or rechargeable batteries) in the case of which the object is to maximize the total amount of usable energy in the energy store before interruption of the supply connection between the energy store and the electrical device to be supplied (minimizing the remaining residual energy). The energy stores may be, for example, lithium, nickel-cadmium or metal-hybrid energy stores. In order to indicate the discharge level of the energy store, that charge state is determined which results from the change in the energy store voltage over time (calculation of the gradient dv/dt). In addition, it is also possible to determine the change in the gradient over time [calculation of the curvature d/dt(dv/dt)].
U.S. Pat. No. 5,166,623 discloses a method for indicating the discharge state of energy stores (batteries), in which, at predetermined times during the discharge phase of the energy store, a characteristic variable which indicates the gradient of a discharge curve of the energy store is in each case determined, in the form of a gradient-related actual value. After this, the gradients determined in this way are compared with one another. The "USED", "EMPTY", and "FULL" states are indicated depending on this comparison result, the "EMPTY" state, which is comparable with the "low energy store" state, being indicated if a determined gradient value is greater than a previously determined gradient value, or if the gradient has increased.
U.S. Pat. No. 5,432,452 and European reference 0 582 527 have disclosed an apparatus for identification of the failure of elements of a battery, in which the second derivative of the discharge voltage curve of the battery is compared with a threshold value. If this specified threshold value is exceeded, then an "elements of the battery have failed" indication is produced.